1. Field of the Invention
The invention relates to the electrolytic precipitation of chromium from hexavalent chromium baths.
2. Description of the Prior Art
Great difficulties result in chomium plating whenever the object has a geometrically complicated shape, since in that case the current density is variable at the surface of the object that is to be chromium plated. At places at which only a low current density prevails, chromium plating is unsatisfactory, because of inadequate coating or no chromium plating at all occurs, whereas at places of very high current density, scorching, nodule formation or wart formation occurs. It is well known that in orcer to overcome these difficulties, conducting salts must be added to the chrome bath or catalysts must be added in the form of sulfates or fluorides or silicofluorides. It is also known that the concentrations and ratios of chromium trioxide and the individual conducting salts must be controlled within very narrow tolerances and further precise temperature control is required. Small tolerances must be maintained for both amounts and for the bath temperature. Beyond that, the deposition of chromium in the case of the known baths still depends very much on the critical proportions of trivalent chromium contained in the electrolytic, the concentration of which changes during the chrome plating process. Consequently, the known galvanic chrome baths must be constantly monitored and checked by means of quantitative analysis of the content of trivalent chromium, of sulfate, or fluoride, or of silicofluoride, and these must be regenerated correspondingly. As a result, the monitoring operation required for the bath is exceedingly expensive in time as well as in energy. Beyond that, chromium plating based on known chromium baths cannot be electrolytically precipitated in a lustrous form directly on to a support consisting of copper or copper alloy. Furthermore, hitherto, it has not been possible with the known crome baths in the case of intricately shaped objects to stainless steel, to fulfill simultaneously the requirements for a high scattering in depth and an avoidance of scorching in a practical and satisfactory manner.
To overcome these difficulties with regard to the formulation and monitoring of the bath, the scattering in depth, the avoidance of scorching, and based on the limited operational possibilities of chrome baths, numerous catalysts or regulating conducting salts have been proposed and used. It is known, for example, to add strontium sulfate to the chrome bath, which causes a self-regulating adjustment of the conductive content in case of a certain critical CrO.sub.3 concentration. Furthermore, the sodium salt of the m-benzene disulfonic acid has also been used (German Pat. No. 1,008,542), in the case of which and due to the low degree of dissolution, and the low solubility at certain CrO.sub.3 concentration, the optimum catalytic conductive content is supposed to adjust itself automatically. In the German published application No. 1,290,782, the addition of 25 to 100 grams per liter of a halogenate organic carboxylic acid with at least three carbon atoms has been described, as a result of which a scattering in depth and the luster of the precipitate as well as the stability of the bath are supported to be improved.
In the German published application No. 1,243,937, a chrome bath is given on the basis of Cr(VI) compounds which contains as a catalyst sulfuric acid as a sulfate ion producing compound and a sulfonated pyridine compound. By the addition of pyridine 3-sulfonic acid, the development of bumps or warts during the precipitation of thick chrome layers is said to be avoided. It is noted that the sulfonic acid is totally different in structure than a quaternary pyridine salt.
Dispite these additives, the known baths in the case of continuous use and operation require a practically daily quantitative analytic supervision and refilling. In addition, the scattering in depth of these known chrome baths is still capable of improvement.